Cardiac output measurements are one of the most important physiological indices of cardiovascular function. Although there is interest in the measurement, there are no relatively inexpensive, noninvasive cardiac output monitors available commercially. We are proposing to develop such a monitor, based on a new instrument which we call the Vector Doppler Ultrasound Velocimeter (VDUV). We have three specific aims which we believe will demonstrate the feasibility of our approach: (1) design and fabrication of a VDUV scanhead with one transmitter and two broad-beam receivers which view the aortiC blood velocity vector through the left parasternal-intercostal spaces, and design of a VDUV esophageal probe to measure aortic blood velocity from within the thoracic cavity, (2) testing in vitro (with steady and pulsatile laboratory flows) and in vivo (five normals and one ICU patient), and (3) preliminary assurance of clinical acceptance through interviews and reports of clinician's assessment after use of the VDUV prototype. Applications for the VDUV cardiac output monitor include use in the operating room, intensive care unit, cardiac catheterization laboratory, and exercise and stress testing clinics.